In general, the liquid-tightness and the degree of coupling have been recognized as the most important factors in connecting hydraulic pipes with each other. Therefore, most of the couplings for connecting pipes, which have various shapes such as those of a linear tube, a bent tube, and a T-shaped tube, are screw-coupled with the hydraulic pipes and include packing members such as O-rings inserted in jointing portions, in common.
However, the usual coupling is assembled with the hydraulic pipes by means of a screw joint, in which case a strong jointing force is required and separate tightening tools are necessary in assembling the pipes with each other. Moreover, when the joint is not firmly maintained, the liquid-tightness can be deteriorated, so that liquid can leak through joint portions, and thereby the reliability on the connection between the pipes are deteriorated.